In recent years, active studies and developments have been carried out so as to utilize liquid crystalline compounds as optical materials, and many of them have been already put into practical use. In order to use a liquid crystalline compound as an optical material, it is essential that after the molecules of the compound are aligned and fixed in a liquid crystal state, the aligned state can be retained under practical use conditions. As methods for making a liquid crystalline compound retained in an aligned state, there have been proposed various methods using polymerizable liquid crystalline compounds, polymeric liquid crystalline compounds, and polymeric liquid crystalline compounds having crosslinkable reactive groups.
Japanese Patent Laid-Open Publication No. 11-080081 discloses a method using polymerizable liquid crystalline compounds, such as those having a mesogen portion comprising two or more benzene rings or similar rings, spacer portions each comprising a hydrocarbon chain, and radically polymerizable reactive groups such as (meth)acrylate groups at one or both of the terminal ends. In this method, a polymerizable liquid crystalline compound is coated in a heat-melted state or in the form of a solution on an alignment substrate and dried if necessary so as to be formed into a liquid crystalline layer. Thereafter, the liquid crystalline layer thus formed is aligned in a liquid crystal state by heating and then polymerized by photo-irradiation so as to fix the layer in the liquid crystal state. However, it is necessary for this method to suppress an undesired polymerization inhibition effect caused by oxygen in the air and conduct some complicated operations such as a photo-irradiation under an inert gas atmosphere, requiring improvements in facilities and apparatus. Since a (meth)acrylate group is apt to polymerize with light or heat, a careful attention must be paid during the synthesis.
Japanese Patent Laid-Open Publication No. 11-158258 proposes a method using liquid crystalline polyesters excellent in an ability to retain an aligned liquid crystal state. However, due to the wide-spread of mobile communication tools, optical films formed from the liquid crystalline polyesters are demanded to have such a retaining ability under more sever conditions and to be more excellent in mechanical strength.
As methods using polymeric liquid crystalline compounds having crosslinkable reactive groups, Japanese Patent Laid-Open Publication No. 09-003454 proposes a method wherein polymerizable reactive groups are introduced into the main chain of a polymer and a method wherein monomer units having polymerizable reactive groups are introduced into the side chain(s). However, since the liquid crystallinity of the compounds is lowered in either of the methods, there is a limit to introduce polymerizable reactive groups in such a large amount that the mechanical strength is sufficiently enhanced. Therefore, alternative methods have been demanded.